1. Field of the Invention
The present invention generally relates to a cellular code division multiple access (CDMA) communication system. More particularly, the present invention relates to an non-scheduled transmission method and apparatus for transmitting non-scheduled data through an enhanced uplink dedicated transport channel.
2. Description of the Related Art
A universal mobile telecommunication service (UMTS) system serving as the third generation mobile communication system uses wideband code division multiple access (CDMA) based on a global system for mobile communications (GSM) serving as a European mobile communication system and general packet radio services (GPRS). The UMTS system performs packet-based transmission of text, digitized voice, video, and multimedia at data rates up to 2 megabits per second (Mbps) that offers a consistent set of services to mobile phone or computer users no matter where they are located in the world.
In uplink (UL) communication from a user equipment (UE) to a base station (BS) or Node B, the UMTS system uses a transport channel such as an enhanced uplink dedicated channel (EUDCH or E-DCH) to improve the performance of packet transmission. The E-DCH supports technologies such as adaptive modulation and coding (AMC), a hybrid automatic retransmission request (HARQ), Node B controlled scheduling, a shorter transmission time interval (TTI), and so on to support stable high-speed data transmissions.
The AMC determines modulation and coding schemes of a data channel according to channel status between a Node B and a UE, and improves the efficiency of the resources being used. A combination of the modulation and coding schemes is referred to as a modulation and coding scheme (MCS). Various MCS levels can be defined by supportable modulation and coding schemes. The AMC adaptively determines an MCS level according to channel status between a Node B and a UE, and improves the efficiency of the resources being used.
The HARQ is a scheme for retransmitting a packet to compensate for an erroneous packet when an error occurs in an initially transmitted data packet. The HARQ scheme is divided into a chase combining (CC) scheme for retransmitting a packet with the same format as that of the initially transmitted data packet when an error occurs, and an incremental redundancy (IR) scheme for retransmitting a packet with a format different from that of the initially transmitted data packet when an error occurs.
According to the Node B controlled scheduling, the Node B determines a data rate for an uplink data transmission through an E-DCH and an upper limit of an available data rate, and sends the determined data rate information to a UE. The UE refers to the data rate information, and determines a data rate of the E-DCH to send data.
A shorter TTI is less than the minimum TTI of 10 ms for the conventional DCH, such that a retransmission delay time is reduced and hence high system throughput can be achieved.
FIG. 1 illustrates uplink packet transmissions through E-DCHs in a conventional wireless communication system. In FIG. 1, reference numeral 100 denotes a Node B for supporting E-DCHs, and reference numerals 101, 102, 103, and 104 denote UEs using the E-DCHs. The UEs 101 to 104 transmit data to the Node B 100 through E-DCHs 111, 112, 113, and 114, respectively.
Using data buffer status, requested data rate, or channel status information of the UEs 101 to 104, the Node B 100 provides each UE with information indicating if E-DCH data transmission is possible, or data rate information for controlling an EUDCH data rate. To improve the overall performance of the system, a scheduling operation assigns relatively low data rates to the UEs 103 and 104 far away from the Node B 100 such that a noise rise or rise over thermal (RoT) value measured by the Node B 100 does not exceed a target value. However, the scheduling operation assigns relatively high data rates to the UEs 101 and 102 close to the Node B 100.
FIG. 2 is a message flow diagram illustrating a transmission and reception process through a conventional E-DCH.
Referring to FIG. 2, the E-DCH is established between a Node B and a UE in step 202. This E-DCH setup process comprises a process for transmitting and receiving messages through a dedicated transport channel. In step 204, the UE notifies the Node B of scheduling information. The scheduling information preferably comprises UE transmission power information about an uplink channel status, information about a remaining amount of UE transmission power, information about an amount of data, stored in a buffer, to be transmitted from the UE, and so on.
In step 206, the Node B schedules data transmissions of a plurality of UEs, and monitors the scheduling information of the UEs. In step 208, the Node B makes a determination for allowing the UE to perform uplink packet transmission using scheduling information received from the UE, and sends scheduling assignment information to the UE. The scheduling assignment information comprises information about an allowed data rate and allowed transmission timing, and so on.
In step 210, the UE determines a transport format (TF) of an E-DCH to be transmitted in the uplink direction using the scheduling assignment information. The UE sends information regarding the determined TF to the Node B in step 212, and transmits UL packet data using the E-DCH according to the determined TF in step 214. The TF information preferably comprises a transport format resource indicator (TFRI) indicating resource information necessary to demodulate the E-DCH. In step 214, the UE selects an MCS level while considering a data rate assigned by the Node B and a channel status, and transmits the uplink packet data using the MCS level.
In step 216, the Node B determines if an error is present in the TF information and the packet data. In step 218, the Node B sends negative acknowledge (NACK) information to the UE through an NACK channel if an error is present, or sends acknowledge (ACK) information to the UE through an ACK channel if no error is present. When the ACK information is sent, the packet data transmission is completed and the UE transmits new user data through an E-DCH. However, when the NACK information is sent, the UE retransmits the same packet data through the E-DCH.
The Node B assigns a low data rate to a UE far away from the Node B, a UE in a bad channel status, or a UE for providing a low priority data service, and assigns a high data rate to a UE close to the Node B, a UE in a good channel status, or a UE for providing a high priority data service, thereby improving the performance of the overall system.
The UE enables non-scheduled transmission (referred to as non-scheduled transmission) for transmitting uplink data through the E-DCH without using scheduling assignment information. The non-scheduled transmission can quickly transmit E-DCH data by omitting a series of processes for sending scheduling information from the UE to the Node B and receiving scheduling assignment information from the Node B. The system limits a data rate possible for the non-scheduled transmission to within a relative low level, thereby maintaining system performance enhancement through the Node B controlled scheduling and reducing a delay time due to scheduling.
FIG. 3 illustrates transport format combinations (TFCs) available for an E-DCH to be transmitted through the uplink to control a data rate of a UE in ascending order of E-DCH data rates or power levels.
Reference numeral 301 denotes a TFC set (TFCS) configured by a radio network controller (RNC) or a set of all TFCs available in the UE. Reference numeral 302 denotes TFCs (referred to as a TFC subset) controlled by the Node B within the TFCS 301 configured by the RNC. The UE selects a suitable TFC from the TFC subset 302 while taking into account an amount of data remaining in a buffer, necessary spare power, and so on. A minimum TFC set 303 can be a set of the TFCs possible for non-scheduled transmission. That is, the UE can use TFCs of the minimum TFC set 303 without the Node B's scheduling. The TFC subset 302 is equal to the TFCS 301 or is included in the TFCS 301. Alternatively, the TFC subset 302 is equal to the minimum TFC set 303 or includes the minimum TFC set 303.
Conventionally, because a data rate and a transmission power level have a one-to-one correspondence relation, uplink interference increases as the E-DCH data rate increases. Accordingly, when the E-DCH data rate used for the non-scheduled transmission increases, high uplink interference occurs, resulting in the degradation of system performance. An E-DCH data rate available in the non-scheduled transmission needs to be controlled to within a relatively low value such that the uplink interference due to the non-scheduled transmission is controlled.
In addition to the Node B controlled scheduling, additional signaling is required to control an E-DCH data rate available in the non-scheduled transmission. Conventionally, an allowable signaling overhead ratio is within about 10% when data is transmitted. When 16 header bits of the conventional radio link control (RLC) protocol data unit (PDU) and 16 cyclic redundancy check (CRC) bits are overhead bits, a possible data size is 320 bits, with an overhead of 32 bits, where the overhead ratio is 10%. When a data rate associated with an E-DCH TTI is computed, a data rate is 32 kbps (with an overhead of 320 bits/10 ms) in case of a TTI of 10 ms, and a data rate is 160 kbps (with an overhead of 320 bits/2 ms) in case of a TTI of 2 ms. In case of an E-DCH with the 2-ms TTI, a relatively high data rate is required and high uplink interference occurs. In this case, system coverage may be lower.
Accordingly, a need exists for technology for effectively transmitting non-scheduled transmission parameters for an E-DCH in a state in which the signaling overhead does not exceed a predetermined level during a data transmission interval in the conventional communication system as well as the UMTS system.